Many process control applications are implemented with low cost small computer hardware equipped with input/output (I/O) boards coupled to process equipment to be controlled and/or monitored. As such, a requirement for reliable process control back-up systems is becoming increasingly important to counter the adverse effect of computer software and hardware failure upon the process control application. One method of providing back-up is through the use of redundant computer hardware.
However, a problem is presented when it is required to switch between an active computer and a back-up computer as the back-up computer may not be entirely cognizant of the current state of the process control environment. In order to provide for a transparent "bumpless" switch-over between active and back-up computers the back-up computer must be continually apprised of the current state of the process control environment including changes in process setpoints, changes in on-line and off-line processing instrumentation, etc. Furthermore, for the switch-over to occur automatically the back-up computer must be capable of readily detecting a fault in the operation of the active computer and must also include suitable means for initiating and accomplishing the switch-over without disrupting the process control environment. Prior to the invention disclosed herein no known process control system implemented with relatively small, low cost and redundant computer hardware overcame in a satisfactory manner the foregoing and other problems.
It is therefore an object of the invention to provide relatively low cost computer hardware, such as well-known personal-type computers, for use in reliably controlling processes.
It is a further object of the invention to provide for ongoing processes to continue without interruption while a failed process control computer is taken off-line.
It is a still further object of the invention to provide novel bus switch apparatus for bidirectionally coupling a primary and a back-up computer to process control instrumentation.
It is one more object of the invention to provide for a transparent and rapid switch-over of control between an active process control computer and a back-up computer, a current state of the process control environment being regularly updated in the backup computer by the active computer via a local area network.